The present invention relates to quality control testing of non-volatile memory cells. More particularly, the invention relates to margin testing of single polysilicon process EEPROM cells.
Non-volatile memory cells include EPROM, Flash and EEPROM cells. EPROM and Flash cells are programmed by hot electron injection and erased by exposure to UV radiation and by Fowler-Nordheim tunneling, respectively. However, EEPROM cells are both electrically programmed and electrically erased by Fowler-Nordheim tunneling. Unlike EPROM and Flash cells, the threshold voltage associated with a discharged EEPROM cell is negative because electrons beyond the neutral state may be removed from the floating gate. This electron removal gives the floating gate a net positive charge. As described below, this difference can present a challenge to effective margin testing, particularly of the erase margin, in certain types of EEPROMs.
EEPROM cells may have a variety of configurations. In particular, EEPROMs may be formed from single or double polysilicon processes. A double polysilicon process EEPROM has a polysilicon control gate capacitively coupled to its floating gate. A single polysilicon process EEPROM does not have a polysilicon control gate, but instead has a second heavily doped diffusion implant in the cell's substrate which is capacitively coupled to its floating gate. Margin testing of single polysilicon ("single poly") EEPROMs are the focus of this invention.
A dual row line single polysilicon EEPROM cell 30 is shown in FIG. 1A. The cell includes a single polysilicon floating gate structure 32 which performs three functions. At a first end, a tunnel extension 34 of floating gate 32 acts as an electrode in the two terminal device used for tunneling electrons from a heavily doped N.sup.+ implant 35 (also referred to as a programming Memory Diffusion or MD) through a tunnel oxide 36 (often about 80 .ANG. thick) onto floating gate structure 32. At the other end of this floating gate, a wide area plate 38 is employed as one electrode of a capacitor enabling the floating gate 32 to be raised to a high voltage (e.g., about 6 to 11 volts) by capacitively coupling a programming voltage (e.g., about 9 to 13 volts) from a second electrode 40 (heavily doped N+ silicon, referred to herein as a control gate memory diffusion) through an oxide 42 (often about 180 .ANG. thick). Between these two ends is a section of polysilicon that forms the gate 44 of a read transistor (N2).
The read transistor (N2) is connected in series with a word line transistor (N1) having a gate 46 forming part of a word line (also referred to as a row line) 47. The read and word line transistors separate a sense amp negative (-) input 48 (a source line) from a sense amp positive (+) input 50 (a drain line). Charging the floating, gate 32 by tunneling electrons onto it (through tunnel oxide 36) raises the threshold voltage of the read transistor (EEPROM cell 30 is programmed). This shuts off the channel between the sense amp inputs, even when the adjacent word line transistor is turned on. Tunneling electrons off the floating gate 32 reduces the read transistor threshold voltage to negative values, effectively turning this device on (EEPROM cell 30 is erased). The word line transistor in series then controls the signal path between the two sense amp inputs 48 and 50.
The EEPROM cell is programmed or erased by charging or discharging, respectively, the floating gate 32. In order to tunnel electrons onto floating gate 32, a high voltage must be applied to the control gate memory diffusion 40. At the same time, the write column 56 is grounded and the write column transistor (N3) is turned on by, for example, selecting the second row line 31 of the dual row line cell with, for example, 5 volts. The sense amp (-) input 48 car be biased from about 5 volts to a high voltage to assist tunneling electrons onto the floating gate 32. The voltage on the control gate memory diffusion 40 is capacitively coupled to the floating gate 32 as is the sense amp (-) input 48 voltage. The resulting positive voltage on floating gate 32 is sufficient to cause tunneling onto floating gate 32 through the tunnel oxide 36 where it intersects the floating gate (the tunnel oxide window 36a (shaded)), thereby programming the EEPROM cell 30.
In order to tunnel electrons off floating gate 32, a high voltage must be applied to memory diffusion 35 while ground is applied to the second heavily doped N+ implant (control gate memory diffusion) 40 which underlies and is capacitively coupled to the wide area plate 38. During this process, ground is also applied to sense amp (-) input 48. The application of high voltage to memory diffusion 35 is accomplished through a write column 56 and a write column select transistor (N3) including (i) a diffusion region 54 conductively connected to write column 56 for data input, (ii) a source/drain diffusion 58 electrically connected to memory diffusion 35, and (iii) a gate electrode 60, which is part of row line 31. When a sufficient potential is applied to the gate 60 of the write column select transistor through row line 31 while a write signal is applied through write column 56, electrons can tunnel off of the floating gate 32 to erase the EEPROM cell.
A further description of a typical EEPROM cell and its functional elements is available the publication "EPM7032 Process, Assembly, and Reliability Information Package" available from Altera Corporation of San Jose Calif. That document is incorporated herein by reference for all purposes.
In order for an MOS transistor to conduct, the voltage on its gate must overcome (be greater than) the transistor's threshold voltage (V.sub.th). Generally, the threshold voltage is that gate voltage required to create an inversion layer in the transistor's channel so that it conducts, and is a function of the design and process criteria for the cell. When the MOS transistor is a programmable transistor, such as an EEPROM, there are two gates: The floating gate and the control gate. Such a cell will have two threshold voltages, corresponding to each of its programmed and erased states. The floating gate voltage required to invert the transistor (V.sub.th) does not change for a given cell, but the control gate voltage to invert the transistor (that is, to bring the floating gate to V.sub.th) differs depending upon the charged or discharged state of the cell.
Prior to shipping a non-volatile memory cell product, a manufacturer will generally test the cells to guarantee that each bit has a good margin, and that the bit will maintain its programmed or erased state over the lifetime of the cell. The "margin" is the voltage required on a cell's control gate to cause a change in the state of a bit of memory. As illustrated in FIG. 2, since a programmable cell has two threshold voltages, it will have two margin voltages: One for the programmed state and one for the erased state. In an EEPROM cell, an erased bit will have a lower margin voltage, typically between about -5V to 0V, and a programmed bit will typically have a higher margin voltage, typically between about 3V and 8V.
In normal cell operation, the EEPROM's control gate will typically be set at a value between the programmed and erased ranges, for example 1.7V. For margin testing, however, the control gate voltage is swept through ranges of voltages to determine the cell's threshold voltages. For example, for a charged cell the control gate may typically be swept from about 3V to 8V; and from about -5V to 0V for an erased cell.
In practice, margin testing requires a detector to determine when a margin voltage has been reached. This role is typically performed by a sense amplifier, such as that described with reference to FIG. 1A. A particular margin voltage will correspond to a "trip current" (I.sub.trip), which is that current sufficient to switch the output of the sense amp from one bit state (e.g., output low) to another (e.g., output high). In this way, a sense amp may signal a cell tester when a margin voltage has been reached.
In EEPROM cells, an effective margin testing protocol will need to confirm both the program and the erase margin, and the maintenance of the floating gate's voltage over the lifetime of the cell. As noted above, in a typical margin testing protocol for an EEPROM cell, the voltage on the control gate is swept through the typical margin values of approximately -5V to 0V or 3V to 8V, depending whether the cell is discharged or charged, respectively, until the sense amplifier is tripped, indicating that the margin has been reached. The cell is then subjected to accelerated stressing conditions calculated to simulate the stresses which a cell may be expected to face over its lifetime. The margin is then again determined for the cell. If the margin is different than it was initially, it is an indication that the stresses have caused some charge to move onto the floating gate ("charge gain") or off the floating gate ("charge loss"), for discharged and charged cells, respectively. If the change is great enough, indicating that a guaranteed margin will not be maintained over the life of the cell, that cell is rejected.
From the above general description of margin testing, it should be apparent that in order to test a conventional EEPROM cell's erase margin (i.e., for the lower threshold voltage), the cell's control gate would have to be biased to a negative voltage. In a double poly cell this presents no problem since the control gate is isolated from other elements of the cell. However, in a single polysilicon process EEPROM, it is not possible to bias the control gate to a negative voltage. This is because, as noted above, the control gate in a single poly EEPROM is coupled to an N+ diffusion implant in the cell's P-substrate. In order for current not flow through the substrate, as must be the case for proper operation of the cell, the N-P junction of the implant and the substrate must remain backward biased; therefore, the implant must not bear a negative voltage. If a negative voltage was applied to the control gate implant, as would be required to margin a negative voltage, the N-P junction would be forward biased and charge would flow into the substrate disrupting proper cell operation.
Accordingly, there is a need for improved apparatuses and methods for margining single poly EEPROM bits.